Driving the mystery of Netflix and Marvel’s “Jessica Jones” Season 2 is IGH, a shadowy organization with implied connections to Jessica’s past and present that viewers first encountered in Season 1.

The main characters mention IGH a whole bunch as the second season gets going, but the term “IGH” doesn’t get much explanation in the first few episodes. “Jessica Jones” Season one dropped back in 2015 and there’s a lot to remember from it. So if you haven’t had a chance for a re-watch, read on for a primer on IGH.

IGH, said to be a kind of paramilitary organization, first shows up in Season 1 as part of the story of Will Simpson (Wil Traval), better known as the supervillain “Nuke” in the comics. Simpson was introduced as a New York City police officer who was compelled by the supervillain Kilgrave (David Tennant) to kill Jessica’s adopted sister and best friend, Trish Walker (Rachael Taylor). Jessica stopped Simpson and snapped him out of the mind control — Simpson subsequently struck up a relationship with Trish and became obsessed with stopping Kilgrave.

Simpson later reveals he’s a former special forces operative who took part in a series of IGH’s military experiments, headed by Dr. Kozlov (Thomas Kopache). The research resulted in Kozlov creating drugs called Combat Enhancers, which were given to soldiers to increase their physical abilities, while dampening their pain sensations. The drugs had side effects, though, including blind rage incidents and a loss of control, and Simpson had left the program after such an incident.

Simpson had been off the drugs for some time, but as part of his effort to stop Kilgrave, he returned to Kozlov and IGH’s program to obtain Combat Enhancers. Instead of staying with IGH, Simpson stole some Combat Enhancers and went after Kilgrave himself. The drugs unhinged him and in contrast to Jessica, who wanted to capture Kilgrave alive in order to clear the names of people he’d compelled to commit crimes, Simpson wanted to straight-up execute him. When Kozlov sent an IGH operative to bring Simpson in, Simpson killed him, as well as another murder, and then try to kill Jessica, who stopped him with Trish’s help. The last we see of Simpson, he was being dragged off unconscious by IGH affiliated soldiers.

At the end of Season 1, Trish’s mother Dorothy Walker (Rebecca De Mornay), who adopted Jessica as a tween after her family was killed in a car wreck, showed Trish Jessica’s medical file. It turns out that IGH had paid for all of Jessica’s operations and medical bills while she was in the hospital. Trish hypothesizes that IGH might be responsible for Jessica’s superpowers.

Cut to Season 2. In the first episode, Trish has been busy tracking down more of Jessica’s medical records and she makes an important discover: some of Jessica’s files are incomplete. In fact, 20 days of records are missing between Jessica’s accident and when she regained consciousness. Those missing days, she thinks, are when IGH experimented on Jessica.

Where does this lead? You’ll have to watch the whole second season to find out.

Every Marvel Comics Live-Action TV Show Ranked, from 'Incredible Hulk' to 'Daredevil'

The recent high-profile shows on Netflix, ABC and FX aren't the first ones Marvel has taken to the small screen. Spider-Man, the Incredible Hulk, Blade and even knock-off-ish X-Men had a go before the current golden age. Here are all the Marvel shows, ranked from goofiest pajama costumes to best character-driven dramas.

17. “Spidey Super Stories” (1974-77)Spider-Man takes on some super-boring villains. He goes up against The Prankster (a school principal who keeps pranking his students) and The Spoiler (who just likes to everyday things for people) in these goofy sketches in PBS’ “The Electric Company.” Worth a few minutes of watching on YouTube.

16. "Mutant X" (2001-04)Delightfully 2001 in every way, the syndicated “Mutant X” is the knock-off "X-Men" TV show with a low budget a lot of people probably wanted. With obviously pulled punches and early CGI, “Mutant X” scratches an itch for a brand of superhero show that’s completely rooted in the world of 20 years ago.

15. "The Inhumans" (2017-current)

Nobody seemed able to get over bad wigs and CGI of "The Inhumans" when it first started, and the show didn't improve much from there. Thin plots and boring characters make this the least interesting of all the modern Marvel shows.

14. “Blade: The Series” (2006)Spike TV took the successful “Blade” movies and made a TV show. It’s a decent precursor to the grittier Marvel takes that would later show up on Netflix. Violent, blood-splattered vampire hunting couldn’t hold an audience in 2006, though.

13. “Agents of S.H.I.E.L.D.” (2013-current)For much of its run, “Agents of S.H.I.E.L.D.” has been more or less answering the question, “Hey, what are the normal people up to?” That can be pretty engaging, although keeping the show up with the greater Marvel Cinematic Universe continuity on a budget sometimes reigns it in.

12. "The Defenders" (2017)After four separate, individual series, Netflix and Marvel unite Jessica Jones, Daredevil, Luke Cage and the Iron Fist to save New York in "The Defenders." The culmination of several series' stories and the combination of all four different personalities makes "The Defenders" a lot of fun, but the short series winds up confusing and a bit anti-climactic in its final few episodes.

11. "The Incredible Hulk" (1977-82)Green Lou Ferrigno helped carry “The Incredible Hulk” through five seasons. The formula of David Banner wandering around America, getting into adventures and helping people, is pretty standard for TV. But there’s plenty of wrong-doers to trigger the Hulk’s rage and get him involved — for justice.

10. “Spider-Man” (1978-79)

A pretty deep departure from the Marvel Comics stories, the Japanese take on “Spider-Man” is worth a look just for the way Spider-man pushes a button on a bracelet that shoots out his suit. This version fully understands to folly of trying to make a serious Spider-Man thing in the '70s and goes gleefully the other way with plenty of self-awareness.

9. "The Runaways" (2017-current)

Hulu's first Marvel offering takes a different tack than its other shows. Focusing on a diverse group of teenagers, it's much more about a mystery concerning the parents -- who might, in fact, be evil -- than a bunch of people punching each other.

8. “Agent Carter” (2015-16)

TV gives Marvel the chance to dig into smaller side stories adjacent to its movies. “Agent Carter” takes that idea to its logical extent, turning out a period show led by a woman and Captain America pal. It gets to explore minor characters pretty intimately, while adding depth to the world when it comes time to turn on an “Avengers” movie.

7. "The Gifted" (2017-present)

Spinning off from the realm of "The X-Men," "The Gifted" starts out as a pretty standard superhero story. But with a family angle, some interesting and involving characters, and story topics that go beyond the usual "X-Men" fugitive angle, "The Gifted" is evolving into an interesting new take on the "X-Men" idea.

6. “Luke Cage” (2016-current)

Giving a different perspective on superheroes than anything that’s come before it, Netflix’s “Luke Cage” adds breadth to the MCU that shows how deep and interesting it can be. It’s also very aware of its time and place in culture and doesn’t shy away from the issues inherent in focusing on a bulletproof black man.

5. “Daredevil” (2015-current)

The first of Netflix and more down-to-earth superhero TV shows that take place in shadow of the Marvel Cinematic Universe does a great job of making Daredevil seem like a mostly regular dude. It’s also full of great, visceral fight scenes that pack an intensity that CGI can’t really achieve. And season 3 took the series to a whole new level, elevating it from simply fun to watch to legitimately good.

4. "Iron Fist" (2017-2018)The first season was really, really rough in every way, but season 2 was a whole different animal. It's fun and adventurous in a way none of these other shows are -- basically masterful trash.

3. “Jessica Jones” (2016-current)

Another lower-key superhero story, “Jessica Jones” brings heroes and villains down to their very human flaws. The way it taps into and explores feminist themes gives a whole different take on the idea of superpowers in the Marvel universe.

2. “Legion” (2017-current)

Visually gorgeous, this slow-burn look at one of the “X-Men” universe’s most powerful mutants does, so far, a phenomenal job of exploring its anti-hero. The focus on the mystery of David’s sanity digs deep into what it’s like, and how to cope, with being a mutant in a way that’s very different from other “X-Men” takes.

1. "The Punisher" (2017-current)

The best of the Netflix Marvel shows so far is "The Punisher." Following the exploits of former soldier Frank Castle, it spends most of its time concerned with a bunch of characters struggling to deal with their own personal tragedies. It also has some of the better action scenes Marvel has yet put in its Netflix series.

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There have been more shows based on Marvel Comics than you probably know

The recent high-profile shows on Netflix, ABC and FX aren't the first ones Marvel has taken to the small screen. Spider-Man, the Incredible Hulk, Blade and even knock-off-ish X-Men had a go before the current golden age. Here are all the Marvel shows, ranked from goofiest pajama costumes to best character-driven dramas.